Vol. 10(4), pp. 42-55, August, 2016 DOI: 10.5897/AJPAC2016.0687 Article Number: A0B8D9C60136 African Journal of Pure and Applied ISSN 1996 - 0840 Copyright © 2016 Chemistry Author(s) retain the copyright of this article http://www.academicjournals.org/AJPAC
Full Length Research Paper
Preparation, structural and thermal studies of boroxine adducts having aryl boronic acids and pyrazoles
Hezil Hassan
Department of Chemistry, Iran University of Science and Technology, Narmak, 16846-13114, Tehran, Iran.
Received 2 April, 2016; Accepted 27 May, 2016
PztBu,iPr Four new boroxine adducts ((B3O3(Ph)3PzH) (1), (B3O3(Ph)3( H)2) (2), (B3O3(PhF2)3PzH). PzH (3) and tBu,iPr (B3O3(PhF2)3(Pz H)2) (4)) using phenylboronic acid, 3,5-difluorophenylboronic acid, 1H-pyrazole (PzH) and 3-tert-butyl-5-isopropyl pyrazole (PztBu,iPrH) were prepared and characterized by elemental analysis, IR, 1H-NMR and X-ray diffraction. The crystallographic study reveals that PzH and PztBu,iPrH are bonded to boroxine molecule through B-N dative bond. It also demonstrates the different type of hydrogen bond interactions between adjacent molecules. The thermal stability of these adducts was investigated by TGA.
Key words: Boroxine, crystal structures, hydrogen bonding, thermal study.
INTRODUCTION
Because of importance in different synthetic reactions Davis and James, 2005; James, 2005; Striegler, 2003; and significant applications in diverse areas, boronic Elfeky et al., 2010; Wimmer et al., 2009; Li et al., 2008). acids are of great interest (Phillips and James, 2004; In recent years, boronic acids have also been used as
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Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Hassan 43
HO OH B
tBu,iPr [B3O3(Ph)3PzH] (1) [B3O3(Ph)3(Pz H)2] (2) N N N N H H o o 3:1, Reflux, 70 C HO OH 3:1, Reflux, 70 C B tBu,iPr [B3O3(PhF2)3PzH].PzH (3) [B3O3(PhF2)3(Pz H)2] (4)
F F
Scheme 1. General method for the synthesis of 1-4.
exclusive building blocks in supramolecular chemistry and pyrazoles ligands on the structure and crystal (Fujita et al., 2008; Nishiyabu et al., 2011; Fossey and packing of these adducts. James, 2011). Boronic acids have –B(OH)2 group, and form the six-membered cyclic ring by simple dehydration of boronic acid. It is well established that boron in a cyclic MATERIALS AND METHODS ring (R3B3O3, R = alkyl or aryl group) acts as a lewis acid All synthesis was performed in air, and solvents were used as and has a tendency to accept the lone pair of electrons received. Phenylboronic acid, 3,5-difluorophenylboronic acid, and from the N-donor ligands (lewis base), and involved in the 1H-pyrazole were purchased from Aldrich Chemical Co. 3-tert-butyl- formation of the B-N dative bond in adducts (Icli et al., 5-isopropyl pyrazole was synthesized by previously reported method (Imai et al., 1998). Elemental analysis was carried out on 2011; Höpfi, 1999; Sheepwash et al., 2011, 2013; Jorge 1 et al., 2012). N-donor ligands easily form 1:1 adducts with PerkinElmer Elemental analyzer. IR and H-NMR spectra were recorded on Bruker ALPHA FT-IR and Bruker AM 400 MHz arylboronic acids even under mild reaction conditions, spectrometers, respectively. Thermal analysis was performed on and thermodynamically favored over 1:2 or 1:3 adducts PerkinElmer thermogravimetric analyzer. due to relief in boroxine ring strain. Adducts performing 1:2 and 1:3 boroxine-N-donor ligands stoichiometry are very limited (Beckett et al., 1995; Höpfi, 1999; Kua and Synthesis of adducts 1-4 Iovine, 2005; Domingo et al., 2008; Saha et al., 2013; Adducts 1-4, were synthesized according to scheme 1. Jorge et al., 2016). In 1958, Synder et al. synthesized an adduct by using triphenylboroxine and pyridine by simple warming in anhydrous solvent (Snyder et al., 1958). In Synthesis of 1 2005, Cote et al. investigated highly stable and porous boronic acid derived covalent organic frameworks with A methanolic (10 ml) solution of phenylboronic acid (0.36 g, 3.00 mmol) and PzH (0.06 g, 1.00 mmol) was refluxed at 70°C for 4 h. large surface area (Cote et al., 2005). A large number of Colorless crystals of 1 were obtained by the slow evaporation of boroxine adducts with N–containing compounds have solvent at room temperature in 0.24 g (62.5%) yield. Anal. Calcd for been studied due to wide commercial uses in various C21H19N2O3B3: C, 66.40; H, 5.04; N, 7.37. Found: C, 65.12; H, 4.99; field like flame retardant materials, dopants, in Suzuki- N, 7.17. IR (KBr, cm-1): 3381, 3196, 3012, 2889, 2626, 2317, 2029, 1921, 1814, 1709, 1627, 1461, 1235, 969, 699, 527. 1H-NMR (400 Miyaura coupling reactions, non-linear optical materials, o biosensors, covalent organic frameworks etc. (Bhat et al., MHz, CDCl3, ppm, 25 C): 6.21 (t, 1H, CH, Pz), 7.57 (d, 2H, CH, Pz), 12.61 (s, br, 1H, NH, Pz), 8.03 (dd, 6H, Ph), 7.35 (m, 9H, Ph). 2011; Iovine et al., 2008; Morgan et al., 2000; Mehta and Fujinami, 1997; Yang et al., 2002; Miyaura and Suzuki, 1995; Cote et al., 2005; Türker et al., 2009), but the Synthesis of 2 structural characterization and thermal study of boroxine 2 was obtained in 0.38 g (59%) yield by the same method as adducts with pyrazoles are not reported till now. This tBu,iPr paper presents the synthesis, structural, and thermal applied for 1 using Pz H (0.17 g, 1.00 mmol). Anal. Calcd for C38H51N4O3B3: C, 70.84; H, 7.97; N, 8.69. Found: C, 70.19; H, 7.91; study of four new boroxine adducts. The main purpose is N, 8.49. IR (KBr, cm-1): 3418, 3143, 2956, 2865, 2247, 2139, 2069, to see the effect of substitution in phenyl boronic acids 1971, 1829, 1786, 1609, 1573, 1437, 1296, 1049, 963, 827, 739, 44 Afr. J. Pure Appl. Chem.
Table 1. Crystal structures and refinement parameters for 1-4.
Adducts 1 2 3 4 CCDC 1060378 1060379 1060376 1060377
Molecular formula C21H19N2O3B3 C38H51N4O3B3 C24H17N4O3F6B3 C38H45N4O3F6B3
Mr 379.81 644.26 555.85 752.21 3 Crystal size (mm ) 0.31 ˟ 0.26 ˟ 0.19 0.28 ˟ 0.23 ˟ 0.17 0.23 ˟ 0.17 ˟ 0.11 0.33 ˟ 0.26 ˟ 0.19 Crystal system triclinic monoclinic triclinic triclinic Space group P-1 C 2/c P-1 P-1 a (Å) 10.392(4) 19.887(18) 7.6511(6) 11.821(5) b (Å) 11.741(6) 11.415(11) 11.5133(8) 13.078(5) c (Å) 16.744(8) 18.432(18) 15.4005(11) 14.193(6) α (deg) 84.17(3) 90.00 78.922(4) 84.43(2) β (deg) 89.84(3) 115.16(2) 75.632(5) 68.97(2) γ (deg) 89.88(3) 90.00 77.296(4) 77.50(2) V(Å3) 2032.4(16) 3787(6) 1268.36(16) 1999.1(15) Z 4 4 2 2
ρCalc (gcm-3) 1.241 1.130 1.455 1.250 µ(Mo Kα) (cm-1) 0.081 0.070 0.125 0.097 F(000) 792 1384 564 788 T(K) 296 296 296 296 Theta range for data collection (o) 1.74-28.36 3.56-26.00 1.38-25.00 1.54-25.00 Range of indices -13, 12; -15, 11; -22, 16 -24, 24; -13, 14; -20, 13 -9, 9; -13, 13; -16, 18 -14, 12; -15, 14; -16, 10 No. of reflections collected 9692 2744 4442 6813 Unique reflections 2683 1737 2655 2132 Data/restraints/parameters 9692/0/523 2744/0/224 4442/0/362 6813/0/498 Goodness-of-fit 0.821 0.928 1.135 0.943
Final R indices(I > 2(I) aR1 0.065 0.065 0.061 0.118
bwR2 0.142 0.186 0.174 0.245 a b 2 2 2 2 2 1/2 2 2 2 2 2 R1 = ∑║Fo│─│Fc║ ∕ ∑│ Fo│, wR2 = {∑ (w (Fo ─ Fc ) ) / ∑w (Fo ) } , where w = 1/ (σ (F0 ) + (aP) + (bP)) and P = ( max (0, F0 ) + 2Fc )/3.
1 o 645, 593, 497. H-NMR (400 MHz, CDCl3, ppm, 25 C): 6.51 (s, 2H, 2239, 2069, 1911, 1837, 1755, 1629, 1457, 1213, 913, 729, 653, 1 o CH, Pz), 4.49 (m, 2H, CH, Pz), 1.39 (d, 12H, CH3, Pz), 1.54 (s, 547, 499. H-NMR (400 MHz, CDCl3, ppm, 25 C): 6.53 (s, 2H, CH, 18H, CH3, Pz), 12.64 (s, br, 2H, NH, Pz), 8.03 (dd, 6H, Ph), 7.37 Pz), 4.51 (m, 2H, CH, Pz), 1.42 (d, 12H, CH3, Pz), 1.57 (s, 18H, (m, 9H, Ph). CH3, Pz), 12.69 (s, br, 2H, NH, Pz), 8.07 (dd, 6H, Ph), 7.39 (m, 3H, Ph).
Synthesis of 3 X-ray diffraction analysis 3 was prepared in 0.53 g (64%) yield by using the method as outlined for 1 using 3,5-difluorophenylboronic acid (0.48 g, 3.00 All crystals were obtained from the slow evaporation of methanolic mmol) and PzH (0.06 g, 1.0 mmol). Anal. Calcd for C24H17N4O3F6B3: C, 51.86; H, 2.91; N, 10.08. Found: C, 50.91; H, 2.89; N, 9.97. IR solutions, and mounted on glass capillaries. All data were collected (KBr, cm-1): 3467, 3139, 3089, 2971, 2849, 2597, 2257, 2091, on a Bruker Kappa four circle-CCD diffractometer with graphite- 1969, 1781, 1624, 1579, 1446, 1223, 1321, 1199, 1163, 1077, 929, monochromated MoKα radiation, operated at 50 kV and 40 mA at 1 o 25°C. Data were corrected for Lorentz and polarization effects 829, 644, 547. H-NMR (400 MHz, CDCl3, ppm, 25 C): 6.27 (t, 2H, CH, Pz), 7.64 (d, 4H, CH, Pz), 12.67 (s, br, 2H, NH, Pz), 8.04 (dd, (Sheldrick, 1996), and the SHELXTL program package was used 6H, Ph), 7.40 (m, 3H, Ph). for the structure solution and refinement (Sheldrick 1990, 2000). The hydrogen atoms were placed in geometrically calculated positions by using a riding model, and non-hydrogen atoms were refined anisotropically. Diamond and Mercury softwares were used Synthesis of 4 for the formation of images and hydrogen bonding interactions (Brandenburg, 2000). 1, 3 and 4 are crystallized in triclinic system 4 was synthesized in 0.45 g (59.8%) yield by the same method as with P-1 space group, while 2 in monoclinic system with C2/c space described for 3 using PztBu,iPrH (0.17 g, 1.00 mmol). Anal. Calcd for group. The crystallographic data, hydrogen bond distances, C38H45N4O3F6B3: C, 60.84; H, 5.77; N, 7.46. Found: C, 60.13; H, selected bond lengths and angles are shown in Tables 1, 2, 5.68; N, 7.07. IR (KBr, cm-1): 3418, 3129, 3018, 2917, 2755, 2601, Appendix S1 and S2, respectively. Hassan 45
Table 2. Selected H-Bond parameters for 1-4.
o Bond (symmetry) dD-H (Å) dH···A (Å) dD···A (Å) 2 C17-H17B···B2 0.960 3.141 (15) 3.704 119.1 C17-H17C···B2 0.959 3.358 (8) 3.704 103.7 C18-H18B···π 0.961 3.005 (14) 3.863 149.4 3 C20-H20···B1 0.931 3.403 (7) 3.998 124.0 N4-H4A···F5 0.860 2.337 (7) 3.054 141.1 C4-H4···F3 0.930 2.850 (1) 3.734 159.1 C6-H6···F1 0.930 2.685 (6) 3.272 117.2 C16-H16···F2 0.930 2.470 (6) 3.394 172.6 C19-H19···F6 0.930 2.728 (4) 3.396 129.4 C22-H22···F2 0.930 2.722 (9) 3.351 125.6 C22-H22···F3 0.930 2.558 (7) 3.179 124.6 4 C26-H26C···B3 0.960 3.469 (60) 4.100 125.3 C34-H34B···B3 0.960 3.213 (40) 3.704 113.6 C34-H34C···B3 0.961 3.327 (50) 3.704 105.6 C8-H8···F6 0.929 2.731 (26) 3.540 146.1 C10-H10···F1 0.930 2.818 (18) 3.567 138.3 C12-H12···F2 0.930 2.565 (26) 3.461 161.9 C27-H27C···F4 0.960 2.804 (25) 3.483 128.5 C28-H28B···F6 0.959 2.832 (32) 3.380 117.1 C33-H33B···F5 0.960 2.661 (36) 3.493 145.3 C37-H37A···F3 0.961 2.662 (33) 3.356 129.5 RESULTS AND DISCUSSION difluorotriphenylboroxine and PztBu,iPrH, whereas 1 and 3 are 1:1 adducts of triphenylboroxine and PzH but 3 is tBu,iPr All adducts (B3O3(Ph)3PzH) (1), (B3O3(Ph)3(Pz H)2) crystallized with free pyrazole as solvate. (2), (B3O3(PhF2)3PzH).PzH (3) and tBu,iPr (B3O3(PhF2)3(Pz H)2) (4) have been prepared by using phenylboronic acid, 3,5-difluorophenylboronic acid Infrared and NMR spectroscopy and corresponding pyrazoles (PzH/PztBu,iPrH) in methanol, and the different formulations were confirmed by 1-4 show strong bands in the region 1460-1250 cm-1, and elemental analysis, IR, NMR and crystallographic at 1255 cm-1 due to B-O and B-N stretching bands, structure analysis. 2 and 4 are rare 1:2 adducts of 3,5- respectively (Smith and Northrop, 2014). NH stretching 46 Afr. J. Pure Appl. Chem. Figure 1. (a) Molecular structure of 1. (b) 2-D sheet like framework. Color code: B, green; C, gray; H, purple; O, red; N, blue. bands (3500-3400 cm-1) are shifted at 3100-3055 cm-1 C20-H20···π, 3.502 (54) Å; C21-H21···π, 2.897 (43) Å; due to formation of adjacent B-N dative bond. The IR C40-H40···π, 2.973 (45) Å; C41-H41···π, 2.889 (44) Å; spectra do not show any O-H stretching vibration in the C42-H42···π, 3.510 (37) Å) intermolecular interactions region of 3300-3200 cm-1 that suggests the absence of O- (Sarma and Baruah, 2009) (Appendix Figure S1). The H bands (Faniran and Shurvell, 1968). The formation of angles between weak C-H···B bonds are 150.0 and adducts have also been confirmed by the 1H-NMR 173.5o. All these interactions help to create the two spectra, showing the prominent downfield shift in each dimensional sheet like framework (Figure 1(b)). case with respect to free compounds as shown in Table The asymmetric unit of 2 shows that two boron atoms S3. B(1) and B(1i) have tetrahedral geometry (Sp3), while other boron atom B(2) has a trigonal planar geometry (Sp2). In this molecular structure, B1i, O2i, C1i, N1i lie on Structure description of 1-4 inversion centers with symmetry code (i = 2-x, y, 0.5-z) (Figure 2(a)). From the X-ray structure, it is clear that According to Figure 1a, the crystal structure of 1 shows both PztBu,iPrH ligands in boroxine adduct are anti to each two molecular units. In each unit one boron atom (B(1)) other which is more stable than syn configuration (Iovine has tetrahedral geometry, while other two boron atoms et al., 2008). The B(1)-O(1) (1.437(13) Å) and B(1)-O(2) (B(2) and B(3)) shows trigonal planar geometry. B(1) has (1.446(4) Å) bond distances are greater than the B(2)- Sp3 hybridization due to the additional B-N dative bond. In O(2) (1.362(10) Å). Similarly, the B(1)-C(1) (1.621(8) Å) one molecular unit, the B(2)-O(1), B(2)-O(3), B(3)-O(2) bond length is slightly higher than that of B(1)-C(7) and B(3)-O(3) bond distances are in the range of (1.579(6)Å). The B(1)-N(1) bond length is (1.655(11) Å), 1.347(22) to 1.399(21) Å, and these are much smaller which is greater than the B-N bond of 1. The crystal than that of B(1)-O(1) (1.468(20) Å) and B(1)-O(2) structure analysis describes that one molecular unit is (1.456(23) Å) in B3O3 ring. In the same manner, B(2)-C(7) hydrogen bonded with other units through C17- (1.542(21) Å) and B(3)-C(13) (1.548(24) Å) bond lengths H17B···B2, 3.141(15) Å; C17-H17C···B2, 3.358 (8) Å and are also shorter than the B(1)-C(1) (1.595(25) Å). The C18-H18B···π, 3.005 (14) Å non covalent interactions B(1)-N(1) bond length is (1.617(23) Å) which is nearly (Melikova et al., 2002) (Appendix Figure S2), and the C- matched with the reported literature (Wu et al., 1999). H···B bond angles in 2 are 119.1 and 103.7°C which are Another unit also follows the same pattern as previous lesser than the C-H···B bond angles of 1 (Table 2). Three one. The crystal packing shows that the one unit is non- dimensional zig-zag layered network is obtained by all C- covalently hydrogen bonded to neighboring unit through H···B and C-H···π non covalent interactions (Figure 2b). the various weak C-H···B (C8-H8···B5, 3.153 (58) Å; 3 contains an adduct having 3,5-difluorophenylboronic C16-H16···B5, 3.058 (46) Å; C35-H35···B3, 3.154 (58) acid and PzH with one free PzH in lattice (Figure 3a). In Å), N-HA···π (N2-H2A···π, 3.021 (46) Å; N4-H4A···π, this structure, B(1) has tetrahedral geometry (Sp3), while 2.646 (36) Å) and C-H···π (C19-H19···π, 2.572 (35) Å; B(2) and B(3) both atoms present trigonal planar Hassan 47 Figure 2. (a) Molecular structure of 2. Color code: B, green; C, gray; H, purple; O, red; N, blue. (b) 3-D zig-zag layered network. Color code: color by atomic displacement. Figure 3. (a) Molecular structure of 3. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange. (b) 3-D ladder like framework. Color code: adduct, green; free Pz, blue. 2 geometry (Sp ). The B(1)-O(1) (1.447(5) Å) and B(1)-O(2) 2.850(1) Å; C6-H6···F1, 2.685(6) Å; C16-H16···F2, 2.470(6) (1.447(4) Å) bond distances are greater than the B(2)- Å; C19-H19···F6, 2.728(4) Å; C22-H22···F3, 2.558(7) Å O(2) (1.346(6) Å), B(2)-O(3) (1.378(6) Å), B(3)-O(1) (Appendix Figures S3 and S4(a)). On the other side, the (1.349(5) Å), B(3)-O(3) (1.381(4) Å). B(1)-C(1) bond free PzH in lattice also shows the N4-H4A···F5, 2.337(7) length is 1.608(6) Å, which is slightly higher than the Å and C22-H22···F2, 2.722(9) Å noncovalent interactions B(2)-C(7) (1.566(5) Å) and B(3)-C(13) (1.549(5) Å). B(1)- with adducts (Appendix Figure S4(b)). The C-H···B bond N(1) bond distance is 1.632(5) Å, which is higher than 1 angle in 3 is 124.0° which are lesser than the C-H···B but less than 2 (Appendix Table S1). The packing of bond angles of 1 and greater than 2. All these crystal shows that the molecular units are interconnected interactions design a three dimensional ladder like to each other via various intermolecular hydrogen bond framework (Figure 3b). interactions, that is, C20-H20···B1, 3.403(7) Å; C4-H4···F3, The molecular structure of 4 has same structural 48 Afr. J. Pure Appl. Chem. Figure 4. (a) Molecular structure of 4. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange. (b) 3-D perspective view. Color code: color by atomic displacement. Figure 5. TGA plot for 1-4. dimension and geometry as 2, presented in Figure 4a. H33B···F5, 2.661 (36) Å and C37-H37A···F3, 2.662 (33) Å The B(1)-O(2) (1.438(18) Å), B(1)-O(3) (1.456(17) Å), interactions (Appendix Figures S5 and S6). C-H···B bond B(2)-O(1) (1.461(18) Å) and B(2)-(O(3) (1.407(19) Å) angles in 4 are 125.3, 113.6 and 105.6° which are nearly bond distances are much greater than B(3)-O(1) matched with 2 and 3, but lesser than the C-H···B bond (1.380(17) Å) and B(3)-O(2) (1.363(19) Å). Similarly, the angles of 1. Three dimensional perspective view is B(1)-C(1) (1.607(20) Å) and B(2)- C(7) (1.619(17) Å) created by involving all type of intermolecular interactions bond lengths are higher than that of B(3)-C(13) (Figure 4b). (1.569(20) Å). The B(1)-N(1) and B(2)-N(2) bond lengths are 1.644(25) and 1.658(28) Å, respectively, which are greater than that of 2. The X-ray crystal structural Thermal study analysis shows that the molecule is intermolecular hydrogen bonded to adjacent molecules through C26- All adducts 1-4 are stable at room temperature and their H26C···B3, 3.469 (60) Å; C34-H34B···B3, 3.213 (40) Å; C34- TGA plots are given in Figure 5. 1 and 2 show the one H34C···B3, 3.327(50) Å; C8-H8···F6, 2.731 (26) Å; C10- step decomposition. Adduct 1 decomposes completely in H10···F1, 2.818 (18) Å; C12-H12···F2, 2.565 (26) Å; C27- the temperature range of 169-236°C (~ 92.7% mass H27C···F4, 2.804 (25) Å; C28-H28B···F6, 2.832 (32) Å; C33- loss), while 2 shows the 91.2% mass loss in the Hassan 49 temperature range of 221-377°C. 3 decompose in two of Boroxine Formation from the Aliphatic Boronic Acid Monomers R– steps. In the first step free PzH releases between 103- B(OH)2 (R = H, H3C, H2N, HO, and F): A Computational Investigation. J. Phys. Chem. A 115:7785-7793. 123°C temperature range with 11.4% mass loss, while Brandenburg K (2000). DIAMOND, Version 2.1c, Visual crystal structure the second step corresponds to the removal of adduct in information system, Crystal impact GbR, Bonn, Germany. the 127-223°C temperature range with 76.7% mass loss. Cote AP, Benin AI, Ockwig NW, O’Keefe M, Matzger AJ, Yaghi OM 4 follows the same pattern as 2 with 93.4% weight loss in (2005). Porous, crystalline, covalent organic frameworks. Science. 310:1166-1170. the temperature range of 305-389°C. Davis AP, James TD (2005) in: Schrader, T, Hamilton, AD (ed) In functional synthetic receptors, Wiley-VCH, Weinheim. Domingo SM, Jorge GA, Herbert H (2008). 3-Pyridineboronic acid → Conclusions boroxine → pentadecanuclear boron cage → 3D molecular network: a sequence based on two levels of self-complementary self- assembly. Chem. Commun. 48:6543-6545. 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Sheldrick GM (2000). SHELXTL-NT, Version 6.12, Reference manual, University of Gottingen, Germany. Smith MK, Northrop BH (2014). Vibrational Properties of Boroxine Anhydride and Boronate Ester Materials: Model Systems for the Diagnostic Characterization of Covalent Organic Frameworks. Chem. Mater. 26:3781-3795. Snyder HR, Konecky MS, Lennarz WJ (1958). Aryl Boronic Acids. II. Aryl Boronic Anhydrides and their Amine Complexes. J. Am. Chem. Soc. 80:3611-3615. Hassan 51 APPENDIX Preparation, structural and thermal studies of boroxine adducts having aryl boronic acids and pyrazoles Table S1. Selected bond lengths (Å) for 1-4. 1 O1—B1 1.468(20) O6—B5 1.355(22) O1—B2 1.347(22) O6—B4 1.457(22) O2—B1 1.456(23) N1—B1 1.617(23) O2—B3 1.351(21) N3—B4 1.612(22) O3—B2 1.399(21) C1—B1 1.595(25) O3—B3 1.374(18) C7—B2 1.542(21) O4—B4 1.457(20) C13—B3 1.548(24) O4—B6 1.342(22) C22—B4 1.599(25) O5—B5 1.378(19) C28—B5 1.552(24) O5—B6 1.389(21) C34—B6 1.550(21) 2 O1—B1 1.437(13) B2—O2i 1.362(10) O1—B1i 1.437(13) N1—B1 1.655(11) O2—B1 1.446(4) C1—B1 1.621(8) O2—B2 1.362(10) C7—B2 1.579(6) 3 O1—B1 1.447(5) O3—B3 1.381(4) O1—B3 1.349(5) N1—B1 1.632(5) O2—B1 1.447(4) C1—B1 1.608(6) O2—B2 1.346(6) C7—B2 1.566(5) O3—B2 1.378(6) C13—B3 1.549(5) 4 O1—B2 1.461(18) N1—B1 1.644(25) O1—B3 1.380(17) N2—B2 1.658(28) O2—B1 1.438(18) C1—B1 1.607(20) O2—B3 1.363(19) C7—B2 1.619(17) O3—B1 1.456(17) C13—B3 1.569(20) O3—B2 1.407(19) Table S2. Selected bond angles (deg) for 1-4. 1 B2—O1—B1 122.90(34) O2—B1—C1 113.05(34) B3—O2—B1 121.98(31) O1—B1—C1 111.37(35) B3—O3—B2 121.07(35) O2—B1—N1 104.37(29) B6—O4—B4 122.18(34) O1—B1—N1 103.62(31) B5—O5—B6 120.74(33) C1—B1—N1 110.32(32) B5—O6—B4 121.75(31) O1—B2—O3 119.20(35) C19—N1—B1 123.49(32) O1—B2—C7 121.46(40) N2—N1—B1 130.28(33) O3—B2—C7 119.34(38) N4—N3—B4 121.64(32) O2—B3—O3 120.88(39) C40—N3—B4 131.93(33) O2—B3—C13 120.51(36) C6—C1—B1 122.00(34) O3—B3—C13 118.61(38) 52 Afr. J. Pure Appl. Chem. Table S2. Contd. C2—C1—B1 122.64(35) O6—B4—O4 113.92(34) C8—C7—B2 120.77(42) O6—B4—C22 112.10(33) C12—C7—B2 122.15(43) O4—B4—C22 111.12(35) C18—C13—B3 121.17(41) O6—B4—N3 105.64(29) C14—C13—B3 121.48(40) O4—B4—N3 102.85(31) C27—C22—B4 121.58(34) C22—B4—N3 110.66(32) C23—C22—B4 122.36(34) O6—B5—O5 120.44(39) C33—C28—B5 121.18(40) O6—B5—C28 120.18(36) C29—C28—B5 121.53(40) O5—B5—C28 119.36(38) C39—C34—B6 122.06(43) O4—B6—O5 120.30(36) C35—C34—B6 121.15(41) O4—B6—C34 120.94(40) O2—B1—O1 113.42(34) O5—B6—C34 118.75(38) 2 B1—O1—B1i 124.27(21) O1—B1—C1 111.42(26) B2—O2—B1 121.0(2) O2—B1—C1 113.10(25) C12—N1—B1 135.98(30) O1—B1—N1 102.82(23) N2—N1—B1 116.88(27) O2—B1—N1 105.84(24) C2—C1—B1 123.57(33) C1—B1—N1 108.06(25) C6—C1—B1 120.15(27) O2—B2—O2i 123.17(15) C8—C7—B2 121.49(13) O2—B2—C7 118.41(11) C8i—C7—B2 121.49(13) O2i—B2—C7 118.41(11) O1—B1—O2 114.69(26) 3 B3—O1—B1 121.52(23) O2—B1—C1 112.67(24) B2—O2—B1 121.57(24) O1—B1—C1 112.69(23) B2—O3—B3 120.06(25) O2—B1—N1 104.64(25) C21—N1—B1 123.15(23) O1—B1—N1 104.29(25) N2—N1—B1 130.35(27) C1—B1—N1 106.95(24) C6—C1—B1 121.42(26) O2—B2—O3 120.90(34) C2—C1—B1 121.51(24) O2—B2—C7 120.33(28) C12—C7—B2 121.05(30) O3—B2—C7 118.77(29) C8—C7—B2 119.79(30) O1—B3—O3 120.89(27) C18—C13—B3 121.78(27) O1—B3—C13 119.55(25) C14—C13—B3 119.79(28) O3—B3—C13 119.55(27) O2—B1—O1 114.58(24) 4 B3—O1—B2 120.41(78) O2—B1—C1 112.90(66) B3—O2—B1 121.12(68) O3—B1—C1 111.93(62) B2—O3—B1 123.96(63) O2—B1—N1 105.85(62) C21—N1—B1 134.00(69) O3—B1—N1 104.24(63) N3—N1—B1 119.30(55) C1—B1—N1 107.28(57) C31—N2—B2 135.80(71) O3—B2—O1 115.41(74) N4—N2—B2 117.25(63) O3—B2—C7 111.77(67) C2—C1—B1 120.64(68) O1—B2—C7 111.58(77) C6—C1—B1 122.18(70) O3—B2—N2 104.27(69) C12—C7—B2 118.91(76) O1—B2—N2 106.56(72) C8—C7—B2 125.66(65) C7—B2—N2 106.44(64) C18—C13—B3 122.23(77) O2—B3—O1 122.01(78) C14—C13—B3 119.48(84) O2—B3—C13 118.45(78) O2—B1—O3 113.84(74) O1—B3—C13 119.50(87) Hassan 53 Table S3. Comparison of 1H NMR spectra. Chemical Shifts (δ ppm) S/No. Pyrazoles/Adducts CH(Pz) CH-N(Pz) CH (CHMe2) Me2 (CHMe2) Me3 (CMe3) NH 1. PzH 6.13 7.15 ― ― ― 9.81 2. PztBu,iPrH 5.89 ― 2.96 1.28 1.32 9.05 3. Adduct 1 6.21 7.57 ― ― ― 12.61 4. Adduct 2 6.51 ― 4.49 1.39 1.54 12.64 5. Adduct 3 6.27 7.64 ― ― ― 12.67 6. Adduct 4 6.53 ― 4.51 1.42 1.57 12.69 Figure S1. 1 shows various C-H···B, C-H···π and N-H··· π noncovalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue. Figure S2. 2 shows various C-H···B and C-H···π non covalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue. 54 Afr. J. Pure Appl. Chem. Figure S3. 3 shows various C-H···F non covalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange. Figure S4 (a) and (b). 3 shows various C-H···B and N-H···F non covalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange. Hassan 55 Figure S5. 4 shows various C-H···B non covalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange. Figure S6. 4 shows various C-H···F non covalent interactions. Color code: B, green; C, gray; H, purple; O, red; N, blue; F, orange.